Tuesday, April 1, 2014

An Analysis of Stove Emissions and the Proposed NSPS

Alliance for Green Heat
April 1, 2014

Conventional wisdom says that cat stoves burn cleanest on a low air setting and non-cats burn cleanest on a high air setting.  Conventional wisdom also says that pellet stoves are cleaner than wood stoves.  However, the database used by industry and the EPA to analyze stoves shows that the reality is much more complicated than conventional wisdom suggests.  And, the implications for designing and testing cleaner stoves that hit a 1.3 standard, or whatever number the EPA arrives at, may be significant. 

This analysis shows that stoves in all categories-cat, non-cat and pellet – usually burn dirtiest on their high air setting, which under a weighted average in the existing NSPS, is largely discounted.  Under the proposed NSPS that high burn rate becomes all-important, and will be the focus of most cat, non-cat and pellet stove testing.  This may lead to manufacturers trying to reduce the high air settings to get their stoves to pass.  This in turn could increase start-up emissions and make it harder to get stoves up to temperature quickly.

This also raises the question about whether it makes sense to test stoves at their highest air setting, when homeowners usually use them at the lowest air setting.  With 40% of non-cat stoves emitting the highest emissions in Category 4, should the new NSPS really test them based on Category 4 emissions and completely ignore Category 1 emissions?  We think that weighting of emissions can still make sense under the new NSPS, although the weighting of wood stoves may be different from the weighting of pellet stove emissions, based on data of where homeowners typically use their respective type of stoves. Weighting should at least be considered to include all the test burns required by the NSPS. If four test burns are required, one high and one low and two more at the dirtiest burn rate, they could all be weighted equally. 

This analysis is based on a database of 147 stoves, compiled by the Hearth Patio and Barbecue Association (HPBA) in 2010.  It became public in January of this year when the EPA released it among the scores of documents they used to determine what emission levels to set for wood and pellet stoves in the new NSPS. 

Summary notes:

·      * When the 5G correction factor is removed, as it will be under the new NSPS, more stoves will likely be able to pass stricter limits than what is often being reported.  Many who cite figures about how many stoves can pass the new NSPS limits appear to be unaware of this.

·     *  Five stoves – 2 catalytic, 2 non-catalytic and 1 pellet–appear to be able to pass a 1.3 gram per hour (g/hr) standard with the 5G correction using crib wood. More will likely pass without it. 

·      * If the EPA settled on a 2 g/hr standard, 16 stoves would appear to pass: 8 pellet, 4 catalytic and 4 non-catalytic.

·     *  On average, non-cat stoves tend to burn cleanest on Category 3, the medium high burn rate and dirtiest on Category 4, the high burn rate.

·     *  Cat stoves burn consistently cleanest on Category 1, the low burn rate and dirtiest on Category 4.

·     *  Pellet stoves burn almost equally cleanly on Categories 1, 2 and 3, but are significantly dirtier on Category 4.

·      * To pass future standards, many manufacturers may look to reducing the air in Category 4, which could have an impact on efficiency and may have effect of reducing maximum BTU output.

·     *  By testing at its dirtiest burn rate, many wood stove would be tested at burn rates which the consumer does not often use.  This may make testing less characteristic of real world use, rather than more.

·      * There is a very slight negative correlation between firebox size and emissions with non-cat stoves showing slightly lower average emissions from larger fireboxes.  Larger non-cat stoves are known to be harder to tune and get to pass.

·      * There is a very slight positive correlation between firebox size and emissions with catalytic stoves showing slightly higher average emissions from larger fireboxes.

·      * On the EPA list of certified stoves, there is a high concentration of stoves that tested just less than 4.5 g/hr, and relatively few that tested above 4.5.  This may indicate that stove companies are able to fine-tune their stoves to hit stricter emission targets.

The Database

The main database used by industry and the EPA has extensive details about 147 stoves, showing emission rates at the 4 burn levels.  It was compiled by Bob Ferguson, a consultant for HPBA who collected data from manufacturers who agreed to share it. The EPA independently also has this data and more.  Legally, emissions data is not protected by the confidential business information (CBI) label, but all manufacturers submit it as CBI, which then requires the EPA to undergo a lengthy process to remove the label. The data was compiled in 2010, so it’s a bit out of date and not an exhaustive list, but it still provides useful and important data to understand how a 1.3 g/hr standard may impact the stove industry in 5 or 8 years, when and if that standard becomes law.  There are up to 100 stoves not on this list, including many that became certified after 2010, which are being analyzed by the EPA. 

5G, 5H and the Correction Factor

One of the biggest hidden features of the NSPS is that stoves at 3.7 or 4.4 g/hr under the existing NSPS could come out a gram or even 2 grams less under the new NSPS.  This is because the EPA is getting rid of an adjustment or correction factor that has been used for stoves tested under certain test methods.  Most of the industry does not realize this and it makes the numbers proposed by the EPA appear stricter than they actually are.  In this analysis here, we are just using the numbers in the existing NSPS and we have not re-adjusted them. If we had, it would make many of them show much lower emission numbers. (For more info on this, see the emission testing method discussion here.) Seventy-two of the 147 stoves on this list used some version of 5G (5G1, 5G2 or 5G3). Twenty-nine did not specify whether they used 5G or 5H.

Stoves that can meet 1.3 grams per hour

This analysis is solely about meeting 1.3, or whatever standard the EPA arrives at, using crib wood testing.  Crib wood testing will continue to be used for five years before transitioning to cord wood.  Once manufacturers start designing for cordwood testing, stoves may operate in people’s homes more like they were tested in the lab, and become cleaner.

For the next five years, the EPA will maintain a 4.5 standard that average stoves can meet.  The proposed 1.3 standard to take effect in five years purposefully seeks to achieve the best demonstrated technology and get manufacturers to change the design of their stoves.

Of the 147 stoves on the list, 5 of them tested under 1.3 g/hr on low and high rates and are likely to pass the proposed new EPA standards using crib wood – but not necessarily cordwood.  The Alliance commented on how the EPA can set a standard for cordwood.  Two of them are cat stoves, 2 are non-cat and one is a pellet stove.

Overall Average Emission Rankings

The overall average weighted emission rate of all the stoves on the list is 2.01 for pellet stoves, 2.05 for cat stoves and 3.51 for non-cat stoves.  On average, all three categories of stoves were dirtiest on Category 4, which is the highest air setting and the most BTU output.  Cat stoves were the only type that was uniformly cleaner at Category 1 and uniformly dirtier at Category 4.  For non-cats, the cleanest average burn was Category 3, but there was only about a one g/hr difference between all 4-burn rates.  Conversely, cat stoves were consistently and significantly cleaner at one burn rate compared to others, with more than a 2.5 g/hr range.  Pellet stoves were slightly cleaner on Category 2, but Categories 1, 2 and 3 were very similar.  There was about a 1.5-g/hr range between cleanest and dirtiest burns.

It is important to know that EPA emissions numbers do not equally average the 4 burn rates to come up with a final number.  They are a “weighted average,” so that the high burn rate counts for very little and the low burn rate counts a lot.   Since the Category 4 high burn doesn’t affect the weighted average much, most manufacturers don’t pay that much attention to it.   This is one reason why the average Category 4 burns were the dirtiest for all stove types. On the other hand, the low burns are heavily weighted, so stoves are designed to perform well at those levels.  High burns that previously counted for 1-10% of a weighted average will now count heavily.  The good news for stove manufacturers is that high burn emissions are often easier and less costly to reduce than low burn emissions.  But the impact in the field is likely to be increased emissions during start-up.

Its also relevant that the median numbers for each burn rate for each stove type are lower than the average, showing that there are more stoves on the cleaner end of the spectrum. 

Cat Stove Emission Characteristics

Cat stoves were the only stove type to have an average under 1.3 on any burn category.  Cat stoves had an average of 1.1 g/hr on Category 1, the low burn.  But under the new EPA proposal, stoves have to meet 1.3 on their dirtiest setting, which will be Category 4 for cat stoves.  In this stove sample, the average on Category 4 was 3.6.

Under the proposed new method of testing at the dirtiest burn level, stoves that consistently have the greatest range between Categories 1 and 4 may be penalized, and stoves that have the least range may benefit.  While cat stoves have the cleanest burn at their “sweet spot” which helped them pass with flying colors over the past 2 decades, they will lose some of that advantage under the new rule which does not average in the clean burns, much less give them greater weight.

Of the 15 cat stoves:

·            *  13 burned cleanest on Category 1, the low burn rate
·            * 12 burned dirtiest on Category 4, the high burn rate
·            * The highest emitter was 9.7 g/hr on Category 4

Non-cat Emission Characteristics

Non-cats had the dirtiest average weighted burn rate of 3.5 and had the dirtiest average on any single burn category.  They had an average of 4.04 on high burn, and the cleanest average was 2.9 on medium high.  This contradicts the much-repeated conventional wisdom that non-cats are cleanest on their highest air setting and dirtiest on the lowest air setting.  Under the new NSPS, these stoves will usually have to be tested at the highest air setting, which is their dirtiest and have the furthest to come down towards 1.3. This may result in non-cats (and cats) being tested at burn rates that homeowners don’t often use.  Unlike cat stoves, that are consistently cleanest on Category 1 and consistently dirtiest on Category 4, the following table shows that cat stoves do not show any similar consistency:

We question whether this data supports the EPA’s proposal to test on a stove’s dirtiest burn rate.  Another option would be to maintain the weighted average for wood stoves (not pellet stoves) but put a cap on emissions on any test run as the EPA proposes to do with outdoor boilers.  Thus, even if the EPA finalized on 1.3 or 2.0 g/hr, the stove could not emit more than 3 or 4 g/hr on any burn rate.

Out of the 110 non-cats on the list:
·      8 were cleanest on Category 1
·      28 were cleanest on Category 4
·      54 were dirtiest on Category 4, the highest burn rate, and
·      33 were dirtiest on Category 1, the low burn rate
·      The highest emitter was 17.4 g/hr on Category 4

Pellet stove Emission Characteristics

As noted above, the cleanest weighted average included one pellet stoves that emitted under 1.3 g/hr on high and low burn rates. Even though nearly a third of pellet stoves had a weighted average under 1.3, with the weighting removed and testing focused on the dirtiest emission rate, many pellet stoves would have to redesign to get their high burn rate emissions down. This is why many manufacturers are currently getting the pellet stove certified so they have a 5-year certificate and won’t have to retest under the new testing protocol for 5 more years. 

Emissions were virtually flat on Categories 1, 2 and 3 (between 1.65 and 1.8) and about the same number of stoves had their cleanest run on Category 1, 2 and 3.  Pellet stoves were consistently dirtiest in Category 4, where emissions jumped to an average of 2.9. One pellet stove put out 11.9 g/hr on Category 4.  However, many experts believe that pellet stoves have a lot of room for improvement and have been intentionally “de-tuned” to meet the 35 to 1 air to fuel ratio. 

Of the 22 pellet stoves:
·      10 burned cleanest on Category 1, the low burn rate
·      12 burned dirtiest on Category 4, the high burn rate
·      5 burned dirtiest on Category 1
·      The dirtiest emitter was 11.9 on Category 4

There is greater certainty around pellet stoves, as they are not undergoing such a drastic switch in testing methods from crib wood to cordwood.  Their test fuel is not remaining the same however, as the EPA will start to require the use of PFI certified pellets, which may perform slightly different than the pellets used before.

If the EPA were ultimately to set a 2 g/hr standard after 5 years, this database shows that pellet stoves would be the category that shines between 1.3 and 2 g/hr, with about a third of them emitting less than 2 g/hr on all four burn rates.  If the EPA were to set a 2.0 limit, there are hardly any more cat or non-cat stoves that would be less than 2 g/hr four burn runs on this list (more would likely pass after taking away the 5H correction).

Implications for Efficiency

Under the proposed NSPS, there will be no minimum efficiency standard, but manufacturers will have to test and report efficiencies using the CSA 415.1 stack loss efficiency test.  This is a method that measures how hot the flue gasses are coming out of the stack, compared to the heat that was transferred from the stove to the room.  All other things being equal, the hotter the gas coming out the chimney, the less efficient the stove.

Stoves tend to burn more efficiently at lower burn rates and less efficiently at higher burn rates.  To design a stove that meets 1.3, or whatever number the EPA arrives at, on high burn, many manufacturers may focus R&D on lowering their highest burn rate.  Under this scenario, a stove’s efficiency may rise.
However, if efficiency calculations under B415.1 are done using a weighted average of all four burn rates, that data will not even be available and efficiency will only be calculated using high and low burn rate data.
One negative implication of reducing air in Category 4 is that stove start up may be more difficult and could result in greater emissions as Category 4 is most consistently used during the start up period. This problem could be reduced if the operator leaves the door slightly ajar, which is already a very common practice and recommended as an option in some owner’s manuals.  However, this cannot be done during certification testing.
Pellet stoves are likely to see the most rapid rise in efficiencies because many of them are currently exempt because of the 35 to 1 air to fuel ratio exemption.  By using this exemption from certification, manufacturers have penalized the efficiency of many pellet stoves.  By removing that exemption to certification in the proposed NSPS, those stoves will have to reduce airflow through the combustion chamber, which can significantly raise efficiency.  A 5 – 20% rise in efficiency, or more, is possible for many exempt stoves.  This will result in significant fuel savings for thousands of consumers.
Tuning a pellet stove for maximum efficiency could cause problems in stoves that are vented through the wall instead of through the ceiling.  Pellet stoves are tested with a vertical stack set-up and a side vented unit will not have the benefit of that increased draft.  The lower efficiency and higher airflow of some existing pellet stoves can help them in the field when they are side vented.
Cat Stove Emission Correlation
We found that there is a small positive correlation between the usable firebox volume and EPA weighted emissions. This means that as the volume of the firebox increases, so does the emission of the stove in g/hr. The value of R (correlation) is 0.3356. Although technically a positive correlation, the relationship between the two variables is weak (the nearer the value is to zero, the weaker the relationship) using an alpha of 0.05, the correlation is not deemed to be statistically significant. The scatter plot below depicts the slight positive slope.

Non-cat Stove Emission Correlation
 The same correlation between firebox volume and EPA weighted emissions evaluated for non-cat stoves shows a small negative correlation. The sample size is much larger at 110 stoves, and the correlation between volume and emissions has an R-value of -0.2566. This negative correlation means that as the firebox size goes up, the weighted emissions of the stove go down, for a cleaner burn. Although technically a negative correlation, the relationship between these two variables is also statistically weak with an alpha level of 0.05. The graph above shows the downward sloping relationship. Firebox sizes are not as relevant with pellet stoves and very few of 22 pellet stoves on the list even included their firebox size, so we did not perform a correlation analysis for them.

Distribution of Tested Emissions in Non-Cat Stoves
 Using the list of EPA certified stoves that was updated in December 2013, we see a very high concentration of stoves that were tested right under 4.5 g/hr.  Conversely, there are very few stoves tested right above 4.5 g/hr.  This may indicate an ability of stove manufacturers to fine tune their stoves to just barely meet stricter EPA standards.  The EPA stove list does not include the lab where they were tested, so it is not known at which EPA approved labs the testing was done.  Similar emission charts for catalytic stoves and pellet stoves did not show any similar concentration around 2.5 or 4.5 g/rh.  Cat stoves are held to a 2.5 g/hr standard in Washington and Oregon and in some change-out programs.


  1. As proven by the above, wood itself and its burning is just too complicated for anyone to do properly. So the best is just to butt out in crowded areas. Wear layers of clothes in a colder house.

  2. Hey Vic,

    I would like to respectfully disagree. Burning wood is definitely a big challenge, but it can be done in a good and clean way. Although it certainly isn't most of them, a lot of stoves out there can meet these new standards, and new stoves in the works will be able to do it as well.

    It's going to require more work by manufacturers, there is no denying it. But the result is an inexpensive, local, renewable, and clean way to heat our homes. I think that is worth the effort.

  3. John, I applaud you for this rather complicated article regarding efficient burns and the implications for the proposed NSPS. I would hope EPA would test for heating efficiency by using the CSA 415.1 stack loss method and have the results posted for each stove. It is important that customers have this knowledge. I am using this method to test a stove design I am working on. It's simple and revealing. I discovered what your article revealed.....That a burn with reduced draft increased the heating efficiency of my stove. Richard J